I've never come up with a snappy comeback. The best I've managed is "Are you completely retarded or just really special?" Alas, that is an insult to mentally challenged people, and it doesn't really get the point across anyway.
That's why I'm glad Heather at Crooks and Liars finally put it all into reasonably brief words for me:
In Matthew Dowd's world, the left being upset about being lied into war, the spying, the torture, stolen elections, using 9-11 to scare the crap out of the American public, tax breaks for the rich who don't need it, using the Department of Justice as a political arm of the White House and getting a Governor thrown into jail, outing a CIA agent because her husband dared to speak out against Dick Cheney, putting industry hacks in charge of every government oversight agency, and I could go on but I'll stop... being upset about those things is exactly the same as the right wing freaking out over a speech given to school children by President Obama. I don't think so.Yo.